Pocus wrote:when was invented the uppercase / lowercase differenciation?
In Charlemagne's time, as part of the Carolingian Renaissance.
What we know today as 'lower case letters' were developed in late Roman times as a way of writing on paper using a pen, that was easier than the square 'Roman' capital letters designed to be carved into stone. But for a long time you either wrote in all-capitals, or in the new cursive scripts - and there were lots of different local variations of them, and so by the 8th century AD a monk in France couldn't read Latin text written in Germany, and a monk in Italy couldn't read Latin text written in France, because the scripts used were so different.
Charlemagne thought that was a bad idea, and ordered his ministers to invent a single, clear style of handwriting that would be used for all books, laws and charters throughout his empire. They decided to use cursive ("lower case") letters for the body of the text, but important words such as the first line of each chapter and the first word in each sentence would be distinguished by writing them in Roman ("upper case") letters instead. The rules of capitalisation wouldn't be standardised until many centuries later, but that's when the original idea of mixing the two letter styles deliberately for stylistic effect in the same piece of writing was developed.
We can also thank Charlemagne for the practice of putting spaces between words, insteadofrunningallthewordstogetherwithnogaps.